Trioxane, which is an annular trimer of formaldehyde, is a raw material monomer which is widely used in the production of polyoxymethylene (POM), and production methods thereof have long been established. For example, a method comprising a step of generating trioxane by the action of a non-volatile acid represented by sulfuric acid, phosphoric acid or the like in a liquid phase on a concentrated formaldehyde aqueous solution, a step of boiling this trioxane along with water including formaldehyde, and a step of purifying the above trioxane by extraction with an organic solution or recrystallization is known. However, the reaction equilibrium concentration in the liquid phase is extremely low. Thus, vaporization of the reaction product from the reaction system in order to increase the equilibrium concentration of the trioxane is practiced, but the vaporization of the reaction product containing water is generally accompanied by a very high energy consumption.
Thus, a method of synthesizing trioxane by trimerizing directly in the gas phase from formaldehyde gas has long been studied. As mentioned in the research examples of Non-Patent Document 1, because the equilibrium concentration in the gas phase is higher than the equilibrium concentration in the liquid phase, it is expected that the synthesis of trioxane in the gas phase will provide a higher yield of trioxane than synthesis in the liquid phase, and it is also expected to reduce energy consumption in the manufacturing process.
As an example of trimerization directly in the gas phase from formaldehyde, in Patent Documents 1 to 4, upon reacting formaldehyde gas in the gas phase, as the supply source of the formaldehyde gas which is the raw material thereof, methods of pyrolysis of paraformaldehyde, and α-polyoxymethylene, or vaporization of a formaldehyde aqueous solution are mentioned. However, because water is present in the raw materials at a level of several % to several tens of %, this is no only disadvantageous with respect to the gas phase reaction, but there are also the problems that deactivation of the catalyst by large amounts of water, or repolymerization of the formaldehyde gas which is the raw material (paraform conversion), and the like, may readily occur.
Further, as the catalyst used for trioxane gas phase synthesis, the use of solid support bodies of various metal oxides, sulfides, and halides, or metal sulfates, phosphates, organic sulfonates and the like, and further, a support body of a silica gel or the like of a heteropoly acids, and solid acid catalysts of sulfonate type cation exchanged resins or the like, have been proposed (for example, refer to Cited Publications 1 to 4). However, there are the problems that metal oxides or sulfides, halides, or various metal salts may generate basic points or the like due to the presence of impurities deriving from other metal components, and side reactions are readily induced, and a satisfactory yield or selectivity of the trioxane will not necessarily be obtained. Further, due to the problem that heteropoly acids themselves are readily reduced, a strong acid strength and the like, deterioration of the catalyst or coking phenomena due to adhesion of organic matter may readily occur, and discoloration of the catalyst itself after the reaction can be notable. Further, solid acid catalysts represented by cation exchanged resins are useful, but also have shortcomings such as generally having low heat resistance.
As other catalysts, the use of vanadyl hydrogenphosphate semihydrate has also been proposed (Patent Document 5). However, for vanadyl hydrogenphosphate semihydrate, in consideration of the trioxane gas phase equilibrium concentration disclosed in Non-Patent Document 1, a satisfactory value for the trioxane yield has not been achieved.
Further, a method of trimerizing by a heterogeneous system catalyst gas phase reaction of gaseous formaldehyde having a low moisture content using a solid phosphoric acid catalyst comprising silicon phosphate, boron phosphate, aluminum phosphate, zirconium phosphate, titanium phosphate, zinc phosphate, or phosphate salt mixtures, or mixed compounds thereof has also been proposed (Patent Document 6). This method is advantageous in the point that a value close to the gas phase equilibrium concentration can be obtained, but considering the reaction selectivity to trioxane, it does not necessarily provide a satisfactory result. Further, there are problems such as deposition of carbon substances due to polymerization of formaldehyde (paraform conversion) on the surface of the phosphoric acid catalyst, and the like, and in the point of the long term reaction stability•catalyst life, there is demand to further enhance solid phosphoric acid catalysts.    Patent Document 1: Japanese Examined Patent Application Publication No. S40-12898    Patent Document 2: Japanese Examined Patent Application Publication No. S44-30735    Patent Document 3: Japanese Unexamined Patent Application, Publication No. S59-25387    Patent Document 4: Japanese Unexamined Patent Application, Publication No. S59-134789    Patent Document 5: Japanese Unexamined Patent Application, Publication No. H07-2833    Patent Document 6: Japanese Unexamined Patent Application, Publication No. 2001-11069    Non-Patent Document 1: W. K. Busfield et al., J. Chem. Sci (A) 1969, 2975